1. Field of the Invention
The present invention relates to a high-density magneto-optical recording medium and a reproducing method for information recorded on the medium.
2. Description of the Related Art
A magneto-optical disk is known as a high-density recording medium, and an increase in information quantity gives rise to a desire for higher densities of the medium. While the higher densities may be realized by reducing the space of recorded marks, the recording and reproducing of the marks are limited by the size of a light beam (beam spot) on the medium. When the presence of only one recorded mark in the beam spot is set, an output waveform corresponding to "1" or "0" may be observed as a reproduction signal according to whether the recorded mark is present or absent in the beam spot.
However, when the presence of plural recorded marks in the beam spot is set by reducing the space of the recorded marks, no change in reproduction output occurs regardless of movement of the beam spot on the medium. Accordingly, the output waveform becomes linear and the presence or absence of the recorded mark in the beam spot cannot be identified. To reproduce such small recorded marks having a period smaller than the size of the beam spot, it is necessary to reduce the beam spot to a small size. However, since the size of the beam spot is limited by the wavelength .lambda. of a light source and the numerical aperture NA of an objective lens, the beam spot cannot be sufficiently reduced to a small size.
A reproducing method has recently been proposed magnetically induced super resolution such that a recorded mark smaller in size than the beam spot can be reproduced by the use of an existing optical system. According to this method, the resolution of reproduction is improved by masking other marks during reproduction of one mark in the beam spot. Accordingly, a super resolution disk medium is required to have at least a mask layer or a reproducing layer for masking other marks so that only one mark may be reproduced during signal reproduction, in addition to a recording layer for recording marks.
A magneto-optical recording medium using a perpendicular magnetization film as the reproducing layer is proposed in Japanese Patent Laid-open No. 3-88156. In the prior art described in this publication, however, an initializing magnetic field of about several kOe is required to initialize the reproducing layer. Accordingly, a compact recording apparatus cannot be made. On the other hand, a magneto-optical recording medium using a magnetic film as the recording layer is proposed in Japanese Patent Laid-open No. 5-81717. This magnetic film has an easy axis of magnetization in a plane at room temperature and has an easy axis of magnetization perpendicular to a film surface at a given temperature or higher.
The principle of reproduction in this prior art will now be described in brief with reference to FIGS. 28A, 28B, and 28C. As shown in FIG. 28C, a magneto-optical disk 2 is formed by laminating a magnetic reproducing layer 6 and a magnetic recording layer 8 on a transparent substrate 4. The magnetic reproducing layer 6 has an easy axis of magnetization in a plane at room temperature. However, when the medium is heated to a given temperature or higher by applying a reproducing power, the easy axis of magnetization is changed to a perpendicular direction. The magnetic recording layer 8 is a perpendicular magnetization film. Reference numeral 10 denotes a light beam.
The intensity distribution of the light beam is a Gaussian distribution as shown in FIG. 28A. Accordingly, when the disk is at rest, the temperature distribution on the disk is also a similar distribution such that the central portion is higher in temperature than the peripheral portion. In actuality, however, the disk 2 is rotated in the direction of arrow R shown in FIG. 28C during reproduction. Accordingly, the temperature distribution on the disk in rotation becomes a distribution as shown in FIG. 28B so that a high-temperature area in the beam spot is shifted to the forward direction of rotation of the disk. Owing to such a temperature distribution during reproduction, the easy axis of magnetization of the magnetic reproducing layer 6 becomes an in-plane direction in a low-temperature area in the beam spot. Therefore, the Kerr rotation angle of reflected light becomes almost zero in the low-temperature area. In the high-temperature area, the easy axis of magnetization of the magnetic reproducing layer 6 is changed from an in-plane direction to an perpendicular direction.
The perpendicular magnetization of the magnetic reproducing layer 6 at this time is bonded to the magnetization of the magnetic recording layer 8 by an exchange force, and the direction of magnetization of the reproducing layer 6 is made identical with the direction of magnetization of the recording layer 8, thereby allowing the magnetization recorded in the recording layer 8 to be transferred to the reproducing layer 6. The area size of such transfer can be changed by a reproducing layer beam power or the rotation of the disk. In this manner, the size of the masking reproducing layer is controlled so is to allow the reproduction of only one recorded mark, thereby obtaining the same effect as that in the case of substantially reducing the area of the beam spot to improve the resolution and realize high-density recording and reproduction.
As mentioned above, the intensity distribution of the laser beam 10 directed onto the disk 2 is a Gaussian distribution, and the disk 2 is rotated in the direction of arrow R. As a result, a low-temperature area and a high-temperature area are formed on the reproducing layer 6. The high-temperature area is shifted to the downstream side or the trailing side of the beam spot formed on the disk. In this manner, the high-temperature area where information is reproduced is shifted to the downstream side in the beam spot, so that the intensity of the laser beam is relatively reduced. Thus, the prior art described in Japanese Patent Laid-open No. 5-81717 cannot obtain a large magneto-optical signal output. Further, as an optical mask is formed at only the upstream side in the beam spot, an opening for reproducing information cannot be reduced in size.